1. Field of the Invention
The invention relates to electric lamps and particularly to electric discharge lamps. More particularly the invention is concerned with dimmable, mercury free electric discharge lamps.
2. Description of the Related Art Including Information Disclosed Under 37 CFR 1.97 and 1.98
Commercially available metal halide discharge lighting is efficient and provides reasonably good color rendering and a pleasant white output at rated power. The typical lamp chemistry sealed in the arc tube contains combinations of metal halide salts to optimize the efficiency and color of the output, as well as mercury for obtaining proper voltage and thermal characteristics.
For energy savings, ambience enhancement, and other reasons it is desirable to dim the output of the lamp, as is readily done with incandescent lamps. However, as a typical metal halide lamp is dimmed, the cooler condensate temperatures result in a reduction in vapor pressure of the metal halides, which can affect the balance of individual metal halides causing drifts in the chromaticity in the light output. For example, atomic thallium emission, which is green, can become more evident. There is also often increased atomic mercury emission relative to other species, which can result in a further undesirable green contribution to the output. The depth of dimming may also be limited by lamp instabilities at low power. Though typical metal halide lamps can be dimmed to some extent for energy savings, the color is often poor. There is then a need for high intensity discharge lamps that can be dimmed and still provide consistent color throughout the dimming range.
There have been attempts to improve the dimming behavior of metal halide lamps. Lamp outer jackets can be coated with phosphors to convert mercury radiation to other wavelengths (McAllister in U.S. Pat. No. 4,229,673). Zhu and Maya, in U.S. Pat. No. 6,242,851, describe the use of heat shields to maintain the condensate at sufficiently elevated temperatures down to 50% of rated lamp wattage with a minimal change in color corrected temperature (CCT), although chromaticity data is not reported. Sakai, Okada, Higashisaka, and Hashimoto in U.S. Pat. No. 6,639,341, describe various methods of regulating the condensate temperature. Lambrechts and Maya, in U.S. Pat. No. 6,501,220, describe the use of thallium-free metal halide fills to avoid green emission during dimming from atomic thallium in the discharge.
Hendrix U.S. Pat. No. 6,404,129 discloses the use of high-pressure xenon fills. Spherical or near spherical (bulgy) shaped arc tubes (Sylvania Powerball®) have been disclosed in Lang U.S. Pat. No. 5,936,351. Olsen, Moskowitz, Newell, and Brates in U.S. Pat. No. 6,124,683 have described power modulation at acoustic resonance frequencies for the purpose of straightening arcs in cylindrical mercury-free lamps. References to other examples of acoustic straightening are given in that patent.